1. Field of the Invention
The present invention relates to a method of printing dots of functional material with an inkjet printer at a plurality of intended positions on a substrate according to print data, said inkjet printer comprising a print head containing a plurality of printing elements for ejecting drops of functional material, each printing element characterized by a deviation angle at which a drop of functional material is ejected on the substrate, the method comprising the steps of deriving from the print data a first intended position of a first dot to be printed, determining a first printing element from the plurality of printing elements suitable for printing the first dot at the first intended position, printing the first dot with the first printing element at a first location on the substrate, deriving from the print data a second intended position of a second dot to be printed, determining a second printing element from the plurality of printing elements suitable for printing the second dot, and printing the second dot by the second printing element at a second location on the substrate.
2. Description of Background Art
An inkjet printer may be used to print an image of functional material on a substrate. Such a functional material may be a marking material like ink or another kind of functional material like metal or silicon. Drops of functional material are ejected by a plurality of printing elements of a print head of the inkjet printer on a substrate. The ejected functional material may form an image of dots of functional material like an image of pixels of ink or a mask for a solar cell. Even a three-dimensional image may be printed.
The print head and the substrate are moved relative to one another in at least two directions, for example a main scanning direction and a sub-scanning direction, in such a manner that a location on the substrate determined for a dot of the image according to the digital data is exposed to at least one printing element of the print head.
In industrial applications, images may be printed by means of screen printing which is a very old but renewed printing technique which is widely used for printing structures. A size of an image to be printed in the range of 100 micrometer to 200 micrometer may be achieved by screen printing. Such an image consists of image elements which have to be printed accurately with specified sizes and specified distances for the gaps between the image elements. A gap between two adjacent image elements is typically in the range of 100 micrometer to 200 micrometer wide.
In industrial applications, an image like a solar cell may be printed directly by ejecting metal containing functional material on a substrate in order to create a metal contact as a feature of the solar cell. Solar cell features printed in this way usually have a width larger than 100 micrometer.
However, various applications require even smaller sizes for gaps between adjacent image elements. This can hardly be done by application of screen printing due to throughput and required aspect ratios.
Inkjet technology may be used to produce adjacent image elements with a gap of a smaller size.
Structures with linear features may be more efficient when the width of the linear feature is reduced. For example, instead of directly printing the front-end metal contacts of a solar cell by means of screen printing, at first a mask is printed by an inkjet printer on a solar cell substrate. This masked substrate comprising printed image elements is etched and afterwards the etched parts are plated to create the metal contacts. Using this process, makes it possible to create contacts which are smaller than 100 micrometer, if the gap between the corresponding adjacent printing elements is smaller than 100 micrometer. The smaller the contact is, the larger the area of the solar cell for catching light rays is. In other words, the smaller the contacts on a solar cell, the larger the efficiency of the solar cell may be.
However, every printing element of a print head of an inkjet printer is produced according to specifications with corresponding tolerance ranges. The specifications and the corresponding tolerance ranges may be stored in a control unit of the inkjet printer. They may be stored as a list of characteristics for a standard printing element. A specification may be, for example, an angle of ejection of 0 radians and a maximum angle of deviation of ejecting the functional material, typically up to π/180 radians.
On the other hand, such a specification may also be determined by printing a test pattern that is scanned, resulting in a digital image. By means of the digital image, a characteristic of each printing element like an angle of ejection may be individually determined.
When opposite edges of two adjacent image elements of an image are produced by printing elements having a deviation between 0 radians and π/180 radians, a distance of a gap between the opposite edges may significantly vary. Such a variation in distance between two opposite edges of the image elements due to the maximum of π/180 radians in the angle of deviation may be unacceptable, since image elements may make contact while they are intended to leave a gap between the isolated image elements. On the other hand, an outcome of printing parallel printing elements at a mask for a solar cell may be a plurality of parallel printing elements that are spaced at different distances. Such a variety of distances does not lead to a maximum yield of the solar cell due to localization effects like overheating of a conducting part of the solar cell.